1. Field of the Invention
The present invention relates to a method for forming the field oxide film of a semiconductor device, and more particularly to a method for forming such a field oxide film, which uses different oxidation temperatures at initial and final stages of a field oxidation, thereby preventing a field oxide ungrowth phenomenon and a field thinning phenomenon while achieving an improvement in the characteristics of a finally produced gate oxide film.
2. Description of the Prior Art
A variety of field oxide film formation methods are known. An example of a conventional field oxide film formation method will be described in conjunction with FIGS. 1a to 1e.
FIGS. 1a to 1e illustrate sequential steps of a conventional field oxide film method in which a semiconductor substrate is recessed after the formation of nitride film spacers thereon to form a field oxide film, respectively.
In accordance with this method, a nitride film 13 is first deposited over a semiconductor substrate 11 which has been formed with a pad oxide film 12, as shown in FIG. 1a. The nitride film 13 is then etched at its portion corresponding to a field region by use of an element isolation mask. The etching step is carried out in an overetching manner, so that the semiconductor substrate 11 is recessed to a certain depth, for example, a depth of 50 to 100 .ANG.. Thereafter, a nitride film 14 is deposited over the entire exposed surface of the resulting structure, as shown in FIG. 1b. The nitride film 14 is then dry-etched without using any mask, thereby forming nitride film spacers 14' as shown in FIG. 1c. Using the nitride film spacers 14' as a mask, the exposed portion of the semiconductor substrate 11 between the nitride film spacers 14' is recessed by use of an etch process, thereby forming a hole having a desired depth in the semiconductor substrate 11, as shown in FIG. 1d.
At the silicon etching step for recessing the portion of the semiconductor substrate 11 corresponding to the field region, a portion of the nitride film 13 corresponding to an active region is also partially etched because the etch selectivity of silicon to the nitride film is finite. As a result, a polymer P containing silicon and nitrogen is formed in the hole of the semiconductor substrate 11. The formation of such a polymer is less generated in regions, such as cell regions of a memory device, where the area ratio a of the field region to the active region is relatively small. However, a large amount of polymer P is produced in peripheral circuit regions where the area ratio .alpha. of the field region to the active region is considerably large.
The polymer P formed during the recess silicon etching step is outwardly discharged or left in the hole or valley portion of the field region. In the field region associated with the peripheral circuit region, a relatively large amount of polymer is left on the bottom of the hole. Moreover, the polymer is not simply laid on the bottom of the hole, but embedded to a certain depth in the portion of the silicon substrate exposed in the hole. Accordingly, it is impossible to remove such a polymer using conventional wet cleaning methods.
FIG. 1e illustrates a field oxide film formed in an incompletely grown (namely, ungrown) state when the field oxide film formation method of FIGS. 1a to 1d is used.
As shown in FIG. 1e, when a field oxidation is carried out at a temperature of 1,100.degree. C. just after the processing step of FIG. 1d, namely, in a state in which the polymer P is left on the bottom of the hole in the field region, there is a problem in that a field oxide film is formed in an ungrown state. In FIG. 1e, the field oxide film is denoted by the reference numeral 15.
In order to solve such a problem, various methods have been proposed. One method is illustrated in FIG. 2. In accordance with the method shown in FIG. 2, an additional dry etching step is carried out for the nitride film without using any mask after the etching step for recessing the semiconductor substrate 11, thereby removing the polymer P. When the wet field oxidation is carried out at a temperature of 1,100.degree. C. after the additional dry etching step, a field oxide film is normally grown. Accordingly, the field oxide ungrowth is solved.
In this case, however, a long bird's beak is formed. In other words, the additional dry etching carried out to remove the polymer P also partially removes the nitride film spacers 14'. As a result, it is impossible to suppress a bird's beak penetration during the field oxidation. Due to the additional dry etching step, the manufacturing costs also increase.
Another conventional method for solving the field oxide ungrowth problem is illustrated in FIGS. 3a to 3e. In accordance with this method, a wet field oxidation is carried out after the processing step of FIG. 1d without carrying out the above-mentioned additional dry etching step for the nitride film. FIG. 3a corresponds to the case in which the wet field oxidation is carried out at a temperature of 1,100.degree. C., FIG. 3b shows the case in which the wet field oxidation is carried out at a temperature of 1,050.degree. C., FIG. 3c shows the case in which the wet field oxidation is carried out at a temperature of 1,000.degree. C., FIG. 3d shows the case in which the wet field oxidation is carried out at a temperature of 950.degree. C., and FIG. 3e shows the case in which the wet field oxidation is carried out at a temperature of 900.degree. C.
Referring to FIGS. 3a to 3e, it can be found that a field oxide ungrowth phenomenon occurs in the cases using a field oxidation temperature of 1,050.degree. C. or above whereas a field oxide film 15 is normally grown at a field oxidation temperature of lower than 1,050.degree. C.
This result shows that although the polymer P containing silicon and nitrogen is left after the processing step of FIG. 1d, a normal growth of the field oxide film is achieved when the field oxidation is carried out at a certain temperature or below. That is, although the polymer P is not a material serving as an oxidation barrier, it is thermally activated at a certain temperature or above, so that it is changed into a new, third material capable of serving as an oxidation barrier.
Accordingly, in accordance with the above-mentioned method, it is required to use a field oxidation temperature of 1,000.degree. C. or below in order to change the polymer into a third material, thereby preventing the field oxide ungrowth phenomenon.
FIG. 4 illustrates data about a field thinning phenomenon occurring in the field oxide film structures according to the conventional methods of FIGS. 1a to 1e and FIG. 2, depending on the field oxidation temperature. The field thinning phenomenon is a phenomenon wherein the field oxide film has a reduced thickness in a narrow field region as compared to a wide field region.
It is known that such a field thinning phenomenon is mainly caused by stress accumulated in the field oxide film. Accordingly, the stress existing in the field oxide film should be relieved in order to suppress the field thinning phenomenon. The field oxidation temperature is an important parameter associated with the relief of stress.
Referring to FIG. 4, it can be found that the field thinning phenomenon is more remarkably generated at a lower field oxidation temperature. This is because it is difficult to relieve the stress existing in the field oxide film due to an increased viscosity of the field oxide at a low temperature. For example, in the case using an oxidation window width of 0.20 .mu.m, the thickness of the field oxide film formed at the field oxidation temperature of 1,100.degree. C. is smaller than that at the field oxidation temperature of 950.degree. C. by about 15%.
Consequently, the conventional method involves a problem in that the field oxide film has a reduced thickness in a narrow oxidation window region when a field oxidation temperature of 1,000.degree. C. or below is used, even though it is possible to prevent a field oxide ungrowth at the field oxidation temperature. Where a thin field oxide film is formed, a serious problem associated with an electrical punchthrough phenomenon occurs.
FIGS. 5a and 5b are graphs respectively illustrating test data about characteristics of gate oxide films respectively formed in accordance with the conventional method of FIGS. 1a to 1e or FIG. 2 at different field oxidation temperatures of 950.degree. C. and 1,100.degree. C.
Referring to FIGS. 5a and 5b, it can be found that the gate oxide film formed at a field oxidation temperature of 950.degree. C. exhibits a degradation in the characteristics thereof whereas the gate oxide film formed at a field oxidation temperature of 1,100.degree. C. exhibits superior characteristics. Such results are based on the fact that when a field oxide film is grown at a low field oxidation temperature, it is difficult to relieve the stress existing in the field oxide film due to an increased viscosity of the field oxide, thereby causing the semiconductor substrate to be greatly stressed. Consequently, it is understood that a higher field oxidation temperature is advantageous in terms of the field thinning phenomenon and the characteristics of the gate oxide film.